This invention relates to vehicle wheel alignment systems, and in particular to sensors in a vehicle wheel alignment system which include an active pixel array detector, such as a charge injection device (CID), diode array, or charge coupled device (CCD) array.
Proper alignment of wheels in a vehicle is important for proper handling of the vehicle and proper tire wear. The alignment of a vehicle's wheels is performed primarily by adjusting camber, caster, steering axis inclination (SAI) and toe. Other suspension problems may be detected by examining vehicle ride height (a measure of vertical vehicle body position with respect to some reference such as the ground or a vehicle wheel) and wheel offset distances (a measure of relative horizontal vehicle body position with respect to the vehicle wheels).
It is known that alignment angles may be measured by placing sensor heads on each wheel of the vehicle to form pairs which extend across the front of the vehicle and along each side of the vehicle. Each sensor head has an emitter and a receiver. A sensor head emits a signal which is transmitted to the receiver of the other sensor head of that pair. The receiver converts this signal into a value which is indicative of the corresponding alignment angle of the vehicle. The signal presently used in these sensor heads is in the electromagnetic spectrum at the visual or infrared wavelengths (hereinafter, referred to as light). The light impinges upon a sensing device in the receiver whose output is representative of the measured angle.
Currently, photodiodes, as set forth in U.S. Pat. No. 4,302,104, which is incorporated herein by reference, and linear array type charge coupled devices (CCDs), as set forth in U.S. Pat. No. 5,018,853, are used as the receiver. Each of these devices has inherent limiting factors which affect their suitability for use as detectors in a vehicle wheel alignment system.
The photodiode array has a number of large active areas arranged in a linear array. The angle of the incident light beam is determined by taking the ratio of signals on two adjacent elements of the array. Because the elements are large, the aperture opening has a correspondingly large area. Under certain conditions, diffuse and/or reflected beams can enter the sensor and bias the angular readings. This occurs when a reflected beam and the directly radiated beam are coincident on the array, but their energies are centered at different points. The reflection cannot be identified (and thereby rejected) by the electronics because the large photodiode uses all the energy that is incident upon it (direct and reflected) to produce a signal. Because the reflection cannot be eliminated, a false reading, and hence a wrong angle, will be produced.
The CCD array incorporates many more active elements than the diode array, and those elements are much smaller than the diode array elements. As a result, spatial resolution with a CCD array is greatly enhanced and reflected beams that are co-incident in time but not position with the main beam can be identified and separated before the angle is determined. Problems can still be caused, however, by reflected signals if they are large in amplitude and impinge close to the main beam. These high amplitude signals can lead to signal overload and "blooming." To control blooming, a complex series of controls is needed. Phase interference (speckle) can also occur which distorts the direct beam energy pattern and consequently biases the computed angle. In addition, the reflected beam can cause an over-exposed condition on one or more pixels in the CCD. This will mask the location of the energy peak and no angle can be calculated until the exposure is corrected.
Conventional alignment angle sensors typically have the receiver and emitter in each head positioned close to the axis of the wheel on which they are mounted. But due to mechanical constraints, either the emitter or the receiver may be placed on the axis, but often not both. Errors may be induced into alignment angle measurements due to the off-axis placement. These errors, moreover, are a function of vehicle wheelbase. The wheelbase, however, may not be readily or easily available using prior art systems.
With respect to ride height and wheel offset measurements, it is known to use various mechanical or electro-mechanical devices to determine ride height and/or wheel offset distances. These devices, however, could be improved.